Physicochemical and microbiological quality of rainwater harvested in underground retention tanks

A study was made of the physicochemical properties (among others: pH, temperature, conductivity, hardness, chlorides, dissolved oxygen, ammonia, nitrites, nitrates, manganese, iron) and microbiological quality (coliform, psychrophilic and mesophilic microorganisms) of rainwater harvested from the roofs of three large buildings and from a parking lot, stored in three large underground tanks (with storage volume from 60 m³ to ca. 200 m³), including the variability of the quality. The underground tanks were located in the city of Poznań, and were characterized by different parameters, rainwater sources, rainwater treatment processes, and types of rainwater use. Samples of rainwater harvested in these tanks were collected from March 2019 to February 2020. Physicochemical and microbiological laboratory analyses were performed to determine selected parameters which have an impact on potential treatment and disinfection methods. The results obtained underwent statistical analysis. The laboratory tests showed that the physicochemical quality of rainwater collected in the three underground tanks met the Polish and EU requirements for drinking water standards. The main problem concerns microbiological quality: the number of coliform bacteria reached a value of 19,300 CFU/100 mL, the number of psychrophilic bacteria was over 264,000 CFU/1 mL, and the number of mesophilic bacteria was over 100,000 CFU/1 mL. Also the variability of microbiological quality was very high for all tanks. Analysis of the calcium carbonate equilibrium showed that the harvested water had corrosive properties. The treatment of the investigated rainwater for potable purposes should focus mainly on biological and chemical stability.